Field of the Invention
The present invention relates to techniques for controlling optical modulators, and more particularly, to a technique applicable to a Mach-Zehnder optical modulator.
Description of the Background Art
Optical communication systems have been growing in capacity in response to an abrupt increase in volume of data communication. Semiconductor lasers, which have been widely used as key devices of the optical communication systems, modulate phases or intensities in accordance with transmission distances of optical signals. Small devices are required in optical transmission systems for a middle-and-short distance of 100 km or less, such as metro networks (intra-city communications), and fiber-to-the-home (FTTH) networks. Accordingly, intensity-modulation lasers are widely used. Meanwhile, phase-modulation lasers, which enable both high-speed operation and long-distance transmission, are now widely used in optical transmission systems for a long distance of 100 km or more, such as core networks (inter-city communications).
An example of widely-used phase-modulation lasers is a Mach-Zehnder modulator (hereinafter referred to as MZM). A phase-modulation MZM converts an electric digital signal into an optical digital signal. The phase-modulation MZM varies a refractive index of a multi-quantum well using an electric signal to thus phase-modulate an output of continuous wave (CW) light from a semiconductor laser.
One way to achieve good optical characteristics in the MZM is to reduce an amplitude difference between electric signals that are input to two optical waveguide arms after Y-branching. This is because a large amplitude-difference lowers an optical output, drops an extinction ratio, and intensifies noise. Additionally, a small phase-shift between the electric signals, which are input to the respective optical waveguide arms, reduces signal determination errors after the transmission of the signals.
However, different optical waveguide arms have different electrode patterns before the electric signals enter the optical waveguide arms from input ends of signal electrodes, due to a limited chip-layout of an element. Unfortunately, this causes different wave optical waveguides to have different amplitudes and phases of the electric signals, which are input to the optical waveguide arms.
Adjusting an electrical length in a module substrate on which an MZM element is disposed readily reduces a phase shift between the electric signals. On the other hand, it is difficult to regulate the amplitude difference between the electric signals in response to a change in pattern of the module substrate. This is because low-loss module substrates are widely used for less loss of the electric signals. Accordingly, a well-structured element is necessary for a small amplitude difference between the electric signals in the MZM element.
An exemplary technique for reducing the amplitude difference is described in Japanese Patent Application Laid-Open No. 2014-178480 that provides an input signal adjustment region and adjusts amplitudes of electric signals that are input to two respective optical waveguide arms for equalization.
However, Japanese Patent Application Laid-Open No. 2014-178480 fails to disclose both specific structure of the input signal adjustment region and specific method of the adjustment.